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The crystallization behavior of plagioclase in which An‐rich varieties crystallize at high temperatures and react continuously with the remaining melt to form progressively lower temperature Ab‐rich varieties forms the basis for understanding the meaning of the continuous reaction series of Bowen's reaction series, as discussed in ssss1. Phase stability diagrams summarize what happens when equilibrium conditions are obtained. In the real world, disequilibrium conditions are common so that incomplete reactions between crystals and magmas occur. These are discussed in the section of ssss1 that deals with fractional crystallization.

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Why are phase diagrams important in understanding igneous processes? Several important concepts concerning melting in igneous systems are illustrated in the plagioclase phase diagram.

1 All partial melts are enriched in low temperature components, in this case albite, relative to the composition of the original rock.

2 The smaller the amount of partial melting that occurs in a system, the more enriched are the melts in low temperature constituents such as albite.